The present invention relates generally to modulating array transmitters, and more particularly, to a modulating array transmitter having an on-off keying method that provides for improved power efficiency.
It is known in the art to produce multiple amplitude and phase modulated signals that can assume discrete states of a multi-state signal constellation. Quadrature amplitude modulation (QAM) is one such known technique. Prior devices that produce such signals have typically used traveling wave tube amplifiers, for example.
However, a problem may occur when traveling wave tube amplifiers are used. In order to reduce constellation distortion in applications where constellations are complex and have many possible states (e.g., 64 QAM), traveling wave tube amplifiers must be operated in a high back-off, linear operating mode. However, when operated in this mode, traveling wave tube amplifiers do not perform efficiently. Thus, the use of traveling wave tube amplifiers is not optimum for applications wherein high operating power efficiency is required when generating complex QAM waveforms.
One such application is one involving a satellite-based QAM signal transmitter. In high rate data signal transmission applications, such as satellite applications, wherein it is necessary to transmit microwave signals through severely bandwidth-limited frequency channels, it is a desirable goal to efficiently transmit many data bits per state of a QAM signal constellation.
To solve this problem, the assignee of the present invention has heretofore developed a modulating array transmitter that is disclosed in U.S. Pat. No. 5,612,651, entitled "Modulating Array QAM Transmitter". This modulating array transmitter disclosed has a plurality of saturated phase shift key modulated power stages (comprising a phase shift keyer driving a power amplifier) that produce various amplitude-phase keyed digital signal constellations. For many of the less than full amplitude output power states, the outputs from pairs of power stages are combined so that their output signals cancel. These states require full prime power to drive the power amplifiers and since their outputs do not contribute to useful output power. These symbol states may be achieved by turning off these power amplifier pairs during their symbols, and conserving their prime power.
As for other known prior art, U.S. Pat. No. 5,381,449 issued to Jasper et al. entitled "Peak to Average Power Ratio Reduction Methodology for QAM Communications Systems" discloses a method for improving the ratio of peak power level to average power level in a power amplifier used in a multi-channel, N-level QAM transmitter. U.S. Pat. No. 5,381,449 discloses that the ratio of peak power level to average power level can be reduced by preselecting magnitudes and phase angles of complex-valued pilot symbols used in the transmitter of the QAM communication system. Pilot symbols are preselected that are not part of the well-known constellation of values normally used in the communications system. The pilot values change over time and over channels to reduce the peak to average power ratio in the composite signal produced by the transmitter.
Therefore, it would be advantageous to have a means and methods for improving the efficiency of a modulating array transmitter. Accordingly, it is an objective of the present invention to provide for a modulating array transmitter and method employing on-off keying of amplifiers that improves the power efficiency of the transmitter.